Add constraint to allow only distinct duplicates

Question

I'm not sure if this has been asked before but I can't seem to find it anywhere.

I want to add a constraint to allow only distinct duplicates of two columns:

User       Fob
2          123
2          123
2          456 <<<< NOT ALLOWED

So the last row is not allowed because it's a new combination of distinct values between Users and Fobs.

Answer 1

You can count each combination and get only those having more than 1.

Example:

SELECT
  User,
  Fob,
  count(*) as lines
FROM example_table
GROUP BY User, Fob
HAVING COUNT(*) > 1